Fix hang in buf_flush_set_page_cleaner_thread_cnt

Running mysqld with innodb-buffer-pool-instances > 1 hangs on startup.

On startup wrong variables was being used to detect number of page cleaner
threads. As a result no threads were actually started. And subsequent code
waits for threads to start forever.

Fixed by using page_cleaner->n_workers, which holds number of page cleaner
threads (0 at startup) instead of srv_n_page_cleaners, which holds number
of requested page cleaner threads (4 by default).

storage/innobase/buf/buf0flu.cc

@@ -3487,19 +3487,15 @@ buf_flush_set_page_cleaner_thread_cnt(ulong new_cnt)
 {
 	mutex_enter(&page_cleaner->mutex);
 
-	if (new_cnt > srv_n_page_cleaners) {
+	srv_n_page_cleaners = new_cnt;
+	if (new_cnt > page_cleaner->n_workers) {
 		/* User has increased the number of page
 		cleaner threads. */
-		uint add = new_cnt - srv_n_page_cleaners;
-		srv_n_page_cleaners = new_cnt;
+		uint add = new_cnt - page_cleaner->n_workers;
 		for (uint i = 0; i < add; i++) {
 			os_thread_id_t cleaner_thread_id;
 			os_thread_create(buf_flush_page_cleaner_worker, NULL, &cleaner_thread_id);
 		}
-	} else if (new_cnt < srv_n_page_cleaners) {
-		/* User has decreased the number of page
-		cleaner threads. */
-		srv_n_page_cleaners = new_cnt;
 	}
 
 	mutex_exit(&page_cleaner->mutex);